FIG. 1 depicts a prior art configuration of a WLP copper post-solder ball configuration 100. This prior art WLP configuration 100 comprises a silicon layer 102, which is shown as a portion of a silicon wafer used in the manufacture of integrated circuits. A copper post 104 is connected or electrically bonded to an RDL layer (not specifically shown), which exists between the circuitry of the silicon wafer 102 and the copper post 104. The copper post 104 is used for connecting a circuit on the prior art WLP configuration 100 to a solder ball 108. The solder ball 108 is for electrically and metallurgically connecting the WLP or wafer level package to, for example, a circuit board (not shown).
During the manufacture of a prior art WLP configuration 100, an etching process is used on the copper post 104 which results in the surface 110 of the copper post being embedded or inset 5 to 20 microns below the surface of the epoxy or encapsulant 106, which surrounds the copper post. When the solder ball 108 is reflowed and thereby metallurgically attached to the surface 110 of the copper post 104, an interface between the solder ball 108, the epoxy 106 and the copper post 104 is created, which is shown as the ball/post/epoxy mechanical interface 112. Due to the topology of the ball/post/epoxy mechanical interface 112, wherein a sharp corner of the encapsulant is always in contact with the solder ball 108, drop testing and temperature cycle testing of a prior art WLP part result in WLP part failure that tends to occur at the ball/post/epoxy mechanical interface 112. One of the causes of the failure may be due to expansion and contraction of the encapsulant and the solder ball during a drop test impulse shock or during the heating/cooling cycles of a temperature test. Another possible cause of the failure may originate with the ball/post/epoxy mechanical interface 112 wherein the epoxy transfers heat to the solder ball thereby creating compression and tension stresses thereabout during thermo-mechanical testing and/or field use.
As such, what is needed is a more robust copper post-to-solder ball interface that provides a strong mechanical metallic bond between the metals and provides a level of reliability that consistently passes temperature cycling tests, drop tests and high current tests.